The Institute of Electrical and Electronics Engineers (IEEE) 802.11 supports a power saving transmission mechanism called a traffic indication map (TIM) mechanism. In the TIM mechanism, a station (STA) (namely, a station supporting the 802.11 protocol, referring to a mobile device in this specification) may enter a power saving state, and when a beacon frame transmission cycle of an access point (AP) is reached, the mobile device wakes up to receive a beacon frame, and checks whether indication information included in the beacon frame indicates that the mobile device has downlink data to transmit. The AP allocates an association identifier (AID) to each mobile device when the mobile device establishes a connection with the AP, and the TIM is used in the beacon frame to indicate whether the mobile device has downlink data to transmit. Each bit in an original TIM corresponds to a possible mobile device, and the serial number of the position of the bit (namely, indication bit) in the original TIM corresponds to a transmission indication bit serial number (namely, AID) of a mobile device.
In the existing stipulation of IEEE 802.11, it is defined that one AP can support 2007 mobile devices at most, but actually, the number of mobile devices connected to one AP is limited by the bandwidth of the AP and the like, where generally dozens of, at most, hundreds of mobile devices exist. When allocating transmission indication bit serial numbers to connected mobile devices, the AP generally centralizes the transmission indication bit serial numbers in a certain area. Therefore, the TIM is generally not very long.
In the IEEE 802.11ah standard being stipulated, one AP is required to support 6000 mobile devices. Because IEEE 802.11ah mainly focuses on an application scenario (such as a smart metering scenario) where a mobile device generally has a small data amount, as seen from bandwidth resources, one AP absolutely can support thousands of mobile devices, and sometimes 6000 mobile devices may be on the network simultaneously. As a result, the TIM may be very long. However, the number of bits in the TIM with the value “1” (indicating that downlink data exists) is generally not very large. Therefore, it is necessary and also possible to compress the TIM for sending.
In the existing IEEE 802.11 standard, a simple compression algorithm is defined for the TIM, that is, several leading 0-value bits and several rear 0-value bits in the original TIM are removed directly, so as to reduce the length of the beacon frame. Based on the existing compression algorithm, in a case in which, for example, 1-value bits exist in both the head and the tail, the TIM cannot be compressed even though a large number of 0-value bits exist in the middle. As described above, in main application environments of the current IEEE 802.11, the existing compression algorithm is barely applicable, but in other application environments such as an application environment of IEEE 802.11 ah and an application environment with a large number of STAs, because the existing compression algorithm has low efficiency in compressing the TIM, an actual TIM compression requirement is hard to be met. For other similar indication maps (IMs), the prior art has a similar problem that an actual compression requirement is hard to be met.